Method for preparing glass-ceramic

ABSTRACT

The present invention provides a method for preparing a glass-ceramic containing leucite crystals, comprising the steps of: mixing (1) a glassy material comprising 53 to 65 wt. % of SiO 2 , 13 to 23 wt. % of Al 2 O 3 , 9 to 20 wt. % of K 2 O and 6 to 12 wt. % of Na 2 O and (2) synthetic leucite seed crystals comprising 53 to 64 wt. % of SiO 2 , 19 to 27 wt. % of Al 2 O 3  and 17 to 25 wt. % of K 2 O, and heat-treating the mixture at 750 to 950° C. for 1 to 5 hours; and a dental porcelain powder and a metal-ceramic restoration both comprising a glass-ceramic prepared by the method. The porcelain comprising the glass-ceramic prepared by the method is substantially free of opacification and decrease in the coefficient of thermal expansion, during fusion-bonding to a metal frame.

TECHNICAL FIELD

The present invention relates to a method for preparing a glass-ceramic.The pulverized glass-ceramics (glass-ceramic powders) obtained by themethod of the invention is useful particularly as a porcelain to bebuilt up and fused for coating the surface of a metal frame and therebyfabricating a dental prosthesis with excellent aesthetic qualities.

BACKGROUND ART

A technique is known in which a porcelain powder (hereinafter referredto simply as “porcelain”) comprising a glass-ceramic is built up on andfusion-bonded to the surface of a metal frame to fabricate a dentalprosthesis which has an appearance similar to natural teeth and highmechanical and chemical durability. The metal frame is mainly made of aprecious metal alloy which has an approximately constant coefficient ofthermal expansion (about (14.2±0.5)×10⁻⁶/° C.).

In such dental prosthesis fabrication, a coating process comprisingbuilding up, fusion-bonding and cooling a porcelain on the metal framesurface is repeated several times to form the external shape of thedental prosthesis. Stated more specifically, the fabrication includesthe basic steps of forming three layers by sequentially building up andfusion-bonding an undercoat opaque for concealment of the metal colorand fusion bonding, a dentine porcelain that determines the basic colorof the prosthetic teeth, and an enamel porcelain that reproduces thecharacteristics of tooth enamel. In addition, the fabrication involvesthe step of forming a margin that matches with natural teeth, andadjusting steps such as coloring, color tone modification and the like.Thus, in the known technique, the coating process comprising buildingup, fusion-bonding and cooling a porcelain on a metal frame is repeatedat least 3 times, up to about 10 times.

Therefore, the porcelains are required to have a coefficient of thermalexpansion approximating that of the material of the metal frame and havethermal stability such that their coefficients of thermal expansionscarcely change during the repeated coating process.

Leucite crystals are represented by the chemical formula4SiO₂.Al₂O₃.K₂O(═KAlSi₂O₆). Since leucite crystals have a largecoefficient of thermal expansion, a glass-ceramic containing a specificamount of leucite crystals has a coefficient of thermal expansionapproximately equal to that of the material of the metal frame. Further,when the leucite crystal phase coexists with a glass matrix phase, thetwo phases as a whole have uniform light transmittance (transparency)because the refractive indices of the two phases are close to eachother. Therefore, addition of a coloring ingredient to a porcelaincomprising the coexisting mixture enables desired coloration of arestored dental prosthesis to impart a highly aesthetic appearancesimilar to the appearance of natural teeth. Because of these excellentcharacteristics (coefficient of thermal expansion and transparency) ofleucite crystal-containing glass-ceramics, the use of leucitecrystal-containing glass-ceramics as materials (porcelains) for coatingmetal frames has been proposed.

For example, U.S. Pat. No. 4,604,366 discloses a method for preparing aceramic porcelain, comprising blending a matrix glass with several typesof leucite crystal-containing glass-ceramic frits having differentleucite crystal contents and different coefficients of thermalexpansion. However, this method necessitates troublesome procedures fordetermining the ratio of the at least three types of ingredients andblending these ingredients, in order to suitably control the leucitecrystal content and coefficient of thermal expansion of the finalproduct porcelain. Moreover, the dental prosthesis obtained by firingthe ceramic porcelain has the serious problem of non-uniform leucitecrystal distribution.

U.S. Pat. No. 4,798,536 discloses a method for preparing a ceramicporcelain, comprising mixing a natural feldspar, such as Wyomingfeldspar, as a leucite crystal origin point with a glass matrix-formingingredient, and melting, slowly cooling and then suddenly cooling themixture. However, this method includes troublesome procedures forpurifying the natural feldspar, and is complicated as a whole. Inaddition, trace impurities derived from the natural feldspar are liableto remain in the porcelain and decrease the transparency, therebydeteriorating the color of the resulting dental prosthesis.

When a porcelain is fusion-bonded to a metal frame, a lowerfusion-bonding temperature is desirable from a workability point ofview. However, the leucite crystal phase is instable in “low-fusingporcelain” which is softened and fluidized at 750 to 950° C. andfusion-bonded to a metal surface. Thus, if the porcelain is maintainedat 950° C. or lower, the leucite crystal phase undergoes transformationto a different type of crystal phase, or shifts to a state ofcoexistence with a different type of crystal phase. More specifically,when a powder of a known low-fusing, leucite crystal-containingglass-ceramic (a composite of leucite crystals and matrix phase) isfired at 750 to 950° C. to coat a metal frame, Na—K feldspathic crystals(high temperature-type Na—K sanidine) start to precipitate after a givenperiod of time, and then the leucite crystals begin to decrease andfinally disappear, since the leucite crystals contained are a metastablecrystal phase. The precipitation of the Na—K feldspathic crystals lowersthe coefficient of thermal expansion and causes opacification in theglass-ceramic. Thus, the coefficient of thermal expansion of theglass-ceramic gradually decreases during the repeated coating processcomprising building up, fusion-bonding and cooling of the glass-ceramicon a metal frame. As a result, the glass-ceramic has defects such ascracks owing to the stress of strain between the frame material and theceramic coating layer, leading to low adhesion between the framematerial and the ceramic coating layer. Further, the opacification ofthe glass-ceramic impairs the transparency of the ceramic coating layer.

In the above situation, the development of a novel porcelain which canbe easily prepared and is free of deterioration in characteristics(i.e., decrease in the coefficient of thermal expansion, oropacification) during a process of coating a metal frame is desired.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a leucitecrystal-containing glass-ceramic which can be easily prepared.

Another object of the invention is to provide a leucitecrystal-containing glass-ceramic whose leucite crystal content does notsubstantially change when heated, and which exhibits a stablecoefficient of thermal expansion and excellent transparency, and aporcelain comprising a powder of the glass ceramic.

A further object of the invention is to provide a leucitecrystal-containing glass-ceramic porcelain in which the leucitecrystals, once precipitated, do not substantially decrease in amountduring the process of coating a metal frame.

A further object of the invention is to provide a porcelain in whichcrystals of types other than leucite crystals (e.g., Na—K feldspathiccrystals) do not substantially precipitate during the process of coatinga metal frame, in other words, to provide a leucite crystal-containingglass-ceramic porcelain in which crystals of types other than leucitecrystals (e.g., Na—K feldspathic crystals) begin to precipitatesufficiently long after the precipitation of leucite crystals reachesthe saturation point.

A further object of the invention is to provide a leucitecrystal-containing glass-ceramic porcelain which is free ofopacification and decrease in the coefficient of thermal expansionduring the process of coating a metal frame with the porcelain.

A further object of the invention is to provide a dental prosthesisobtainable by building up and fusion-bonding the leucitecrystal-containing glass-ceramic porcelain onto the surface of a metalframe.

The above objects of the invention can be achieved by mixing a glassymaterial and leucite crystals (seed crystals) previously synthesized,and then heat-treating the resulting mixture under specific conditions.

Specifically, the present invention provides a method for preparing aleucite crystal-containing glass-ceramic, comprising the steps of:

mixing

(1) a glassy material comprising 53 to 65 wt. % of SiO₂, 13 to 23 wt. %of Al₂O₃, 9 to 20 wt. % of K₂O and 6 to 12 wt. % of Na₂O, and

(2) synthetic leucite seed crystals comprising 53 to 64 wt. % of SiO₂,19 to 27 wt. % of Al₂O₃ and 17 to 25 wt. % of K₂O;

and heat-treating the mixture at 750 to 950° C. for 1 to 5 hours.

The glassy material (1) for use in the invention comprises SiO₂, Al₂O₃,K₂O and Na₂O as essential components. Na₂O is a component which lowersthe fusing point of the glassy material (1).

The glassy material (1) may contain optional components which does notprevent precipitation of leucite crystals or inhibit the transparency ofthe glass-ceramic, such as F and colorless oxides of Li, Mg, Ca, Sr, B,P, Ti, Zr, etc. Specific examples of optional components include Li₂O (2wt. % or less), MgO (3 wt. % or less), CaO (3 wt. % or less), SrO (2 wt.% or less), B₂O₃ (3 wt. % or less), P₂O₅ (2 wt. % or less), TiO₂ (3 wt.% or less), ZrO₂(1 wt. % or less) and F (2 wt. % or less). It ispreferable that the total proportion of these optional component(s) inthe glassy material (1) be 6 wt. % or less.

The concomitant use of the optional components accomplishes thefollowing effects. Oxides of Li, Mg, Ca, Sr, B, P, Ti or the like areeffective for lowering the fusing point of the glass-ceramic. Oxides ofMg, Ca, Sr, B, Ti or the like improve the water resistance and acidresistance of the glass-ceramic. Oxides of Mg, Ca, Sr, Ti, Zr or thelike improve the alkali resistance of the glass-ceramic.

In this specification and the appended claims, when an expression suchas, for example, “comprising 2 wt. % or less”, is used to indicate anoxide content, it includes the case where no oxide is contained.

More preferably, the glassy material (1) comprises about 61 to 65 wt. %of SiO₂, about 12 to 20 wt. % of Al₂O₃, about 10 to 15 wt. % of K₂O,about 6 to 10 wt. % of Na₂O, 0.3 wt. % or less of Li₂O, 1.0 wt. % orless of MgO, 2 wt. % or less of CaO, about 0.3 to 1.5 wt. % of B₂O₃, 1wt. % or less of SrO, 2 wt. % or less of TiO₂, 0.5 wt. or less of ZrO₂,0.5 wt. % or less of P₂O₅ and 1.5 wt. % or less of F. In the glassymaterial having the above composition, the total proportion of theoptional components is preferably 6 wt. % or less.

The glassy material (1) can be prepared by a known melting process, forexample by melting a starting mixture comprising predeterminedproportions of components such as oxides, hydroxides, carbonates or thelike, at about 1550 to 1750° C. for about 2 to 5 hours, more preferablyat 1600 to 1700° C. for about 3 to 4 hours. The starting mixture can bemelted in a conventional crucible such as a high alumina crucible, aplatinum crucible, an Rh-containing platinum crucible, a Zr-containingplatinum crucible or the like. Among them, an Rh-containing platinumcrucible and Zr-containing platinum crucible are more preferable.

In producing the glass-ceramic, the glassy material (1) is preferablyused in powder form. A powder of the glassy material (1) can beprepared, for example, in the following manner: a molten glass obtainedby the above melting process is poured into water, or a cruciblecontaining the molten glass is placed in water, to suddenly cool andcoarsely crush the molten glass. The glassy portion is then separated,collected, dried and pulverized to a predetermined size (a particle sizedistribution with usually 200 mesh or smaller (=about 75 um or smaller),more preferably a mean particle size of about 30 to 60 μm), using a rollmill, ball mill, jet mill or like pulverizer, optionally followed bysieving.

In the invention, the synthetic leucite seed crystals (2) for use asseed crystals may be synthetic leucite crystals having a theoreticalcomposition, a leucite solid solution containing SiO₂ dissolved therein,synthetic leucite crystals in which a part (5% or less) of K issubstituted by Rb, or a mixture of at least two of these types ofleucite crystals. When the synthetic leucite seed crystals (2) are usedin a mixture form, the mixing ratio of each component is not limited.Preferably, synthetic leucite crystals having a theoretical compositionare used as seed crystals.

The synthetic leucite seed crystals (2) for use in the inventioncomprise SiO₂, Al₂O₃ and K₂O as essential components. The syntheticleucite seed crystals (2) may contain optional component(s) whichneither decrease(s) the leucite crystal content to 80 wt. % or less, norreduce(s) the transparency. Such optional components include F andoxides of Li, Na, Mg, Ca, Sr, B, P, Ti, Zr, etc. These optionalcomponents reduce a crystallinity in the seed crystals and thus cannotbe used in large proportions, but promote diffusion of the essentialcomponents and achieve a lower melting point of the essential componentswhile melting. A preferable total content of the optional component(s)in the synthetic leucite crystal seed (2) is 3 wt. % or less.

More preferably, the synthetic leucite seed crystals (2) comprise about53 to 56 wt. % of SiO₂, about 22 to 25 wt. % of Al₂O₃ and about 20 to 25wt. % of K₂O.

The synthetic leucite seed crystals (2) are not limited, but may beprepared by a conventional melting process similar to the process forpreparing the glassy material (1). For example, a starting mixturecomprising predetermined proportions of oxides, hydroxides, carbonatesor the like is melted at 1700° C. or higher (more preferably about 1750°C.) for at least 2 hours (more preferably about 3 hours), slowly cooledto about 1300° C. at a cooling rate of about 100° C./hr or slower (morepreferably about 50° C./hr) to complete crystallization, and thenallowed to cool to room temperature (about 15 to 25° C.), thereby givingsynthetic leucite crystals. The formation of synthetic leucite crystalscan be easily confirmed by powder X-ray diffraction analysis of theproduct. In the above slow cooling process, it is preferable that, at anintermediate stage of the slow cooling, the melt is maintained forexample at 1400 to 1500° C. for about 2 to 3 hours. The starting mixturecan be melted in a conventional crucible such as a high aluminacrucible, a platinum crucible, an Rh-containing platinum crucible, aZr-containing platinum crucible or the like.

Alternatively, the synthetic leucite seed crystals (2) can besynthesized by placing the above starting mixture in a crucible andmaintaining the mixture at a temperature not lower than 1400° C. for agiven period of time for firing. In this case, the higher the firingtemperature is, the shorter time the synthesis requires. For example,when the firing temperature is about 1600° C., the mixture is maintainedfor about 5 hours to obtain seed crystals with a crystallinity of about95%. In contrast, when the firing temperature is about 1400° C., themixture needs to be maintained for about 3 to 6 days to obtain seedcrystals having the same degree of crystallinity as above.

Preferably, in production of the glass-ceramic, the synthetic leuciteseed crystals (2) are also used in pulverized form. A powder of thesynthetic leucite seed crystals (2) can be obtained for example asfollows: the crucible containing the high-temperature synthetic leucitecrystals prepared in the above manner is placed in water to suddenlycool and coarsely crush the crystals, and the crystals are separated,collected, dried and then pulverized to a predetermined size (usually aparticle size distribution with 200 mesh or smaller (=about 75 μm orsmaller), more preferably a mean particle size of about 30 to 60 μm)using a pulverizer such as a roll mill, ball mill, jet mill or the like,optionally followed by sieving. Note that the synthetic leucite seedcrystals (2) may be of any size and are not limited to the above sizes,as long as they function as seed crystals. For example, the use ofleucite crystals in the form of fine powder with a mean particle size ofabout 3.5 μm achieves desired effects.

In producing the glass-ceramic of the invention, a starting mixture isused which comprises, per 100 parts by weight of a powder of the glassymaterial (1), 0.5 to 3 parts by weight of a powder of the leucite seedcrystals (2) with a high purity synthesized in the above manner. The useof an excess amount of the leucite seed crystals (2) leads toopacification of the glass-ceramic, and thus is undesirable.

The proportion of the synthetic leucite seed crystals (2) to the glassymaterial (1) is more preferably 1 to 2 parts by weight per 100 parts byweight of the glassy material (1). When using these ingredients in suchproportions, a glass-ceramic can be obtained which has further improvedcharacteristics.

The glass-ceramic of the invention can be prepared by heat-treating theabove mixture usually at about 750 to 950° C. for about 1 to 5 hours,more preferably at about 800 to 900° C. for about 3 to 5 hours.

When the glass-ceramic of the invention is used as, for example, aporcelain, the glass-ceramic is made into a powder by a conventionalpulverizing process, or into a controlled particle size powder bysieving the powder. The powder for use as a porcelain is not limited,but usually has a particle size of 100 μm or smaller. More preferably,the powder has a mean particle size of 5 to 50 μm and contains 1% orless of fine particles with a particle size of 1 μm or smaller.

If the glassy material (1) alone (without the synthetic leucite seedcrystals) is heat-treated, the results are as follows: Even when theglassy material (1) alone is heat-treated, leucite crystals form andslowly grow. This is presumably because the edges of the pulverizedglassy material, or trace amounts of high-melting inorganic impurities(fine particles of Al₂O₃, SiO₂ or the like mixed in duringpulverization) serve as crystal nuclei. However, when the leucitecrystal growth depends on such crystal nuclei formed spontaneously, along period of time is required for the leucite crystals in the glassphase to grow and reach the saturation point. At the time when theleucite crystals reach the saturation point, the glass-ceramic having adesired coefficient of thermal expansion is formed. At that time,however, different types of crystals such as Na—K feldspathic crystals(stable phases) have begun to precipitate. Therefore, it is impossibleto effectively prevent the decrease in the coefficient of thermalexpansion (i.e., opacification) of the glass-ceramic.

On the other hand, the starting mixture for use in the inventioncontains a specific amount of synthetic leucite seed crystals (2), sothat, in the glass phase under heated conditions, leucite crystalsrapidly grow and reach the saturation point in a short time.Accordingly, when the resulting glass-ceramic is used as a porcelain,there is a sufficiently long period of time before different types ofcrystals such as Na—K feldspathic crystals begin to precipitate. As aresult, even when the glass-ceramic is used as a porcelain under hightemperature conditions, the leucite crystals remain stable and theopacification owing to precipitation of different types of crystals(feldspars) is effectively inhibited.

An excessive proportion of the synthetic leucite seed crystals (2) inthe starting mixture used in the invention causes the followingproblems: When the powder mixture of the glassy material (1) and leucitecrystals (2) is heat-treated for sinter-crystallization, the glassymaterial (1) powder and seed crystal (2) powder are not fullyfusion-bonded to each other and leave minute voids, leading toopacification. Moreover, when the glass-ceramic porcelain powder isfusion-bonded onto a metal frame, it is difficult to degas theporcelain, resulting in formation of fine cracks in the fusion-bondedobject, which is not practical.

The leucite crystals in the glass-ceramic obtained by the methodaccording to the invention have a particle size up to 200 mesh (up toabout 75 μm). It is thus apparent that leucite crystals derived from theseed crystals (2) maintain their original particle size. On the otherhand, leucite crystals which have grown using the seed crystals (2) asthe crystal origin point have a mean particle size of about 5 μm orsmaller, mainly because (a) they grow from a number of pieces of crystalorigin point, and (b) the crystal growth rate is low since the heattreatment for crystal growth is carried out at a low temperature of 750to 950° C. Accordingly, there is a state that creates a tensile stressbetween the leucite crystal phase (coefficient of thermal expansionα=about 22×10⁻⁶/° C.) and the matrix glass phase (α=about 9×10⁻⁶/° C.)in the glass-ceramic obtained by crystallization treatment, owing to thedifference between the coefficients of thermal expansion of the twophases. When this ceramic-glass is pulverized, portions with largertensile stress, i.e., portions in which the particle size of leucitecrystals is larger, are selectively crushed. As a result, even when theglass-ceramic porcelain powder has a particle size up to 200 mesh (up toabout 75 μm), the leucite crystal particles with a large particle sizederived from the seed crystals (2) are remarkably decreased.

The glass-ceramic obtained by the invention contains leucite crystals,and has a coefficient of thermal expansion of (12 to 17.5)×10⁻⁶/° C. at50 to 500° C. The glass-ceramic of the invention has the characteristicthat, even if heat-treated under severe conditions (at 850° C. for 3hours or at 750° C. for 10 hours), the glass-ceramic is substantiallyfree of reduction in leucite crystal content or precipitation ofdifferent types of crystals such as Na—K feldspathic crystals.

The leucite crystals in the glass-ceramic obtained by the method of theinvention may be leucite crystals having a theoretical composition, aleucite solid solution containing a SiO₂ component dissolved therein,leucite crystals in which a part (2% or less) of K is substituted by Rb,or a mixture containing at least two of these types of crystals.

In the glass-ceramic obtained by the invention, leucite crystals in anamount equal or close to the saturation amount at the crystal growthtemperature during the production of the glass ceramic are uniformlydispersed as a metastable crystal phase having a mean particle size of10 μm or smaller (more preferably 5 μm or smaller).

The leucite crystal content in the glass-ceramic according to theinvention is about 15 to 43 wt. %, although depending on the proportionsof the glassy material (1) and the synthetic leucite seed crystals (2),heat treatment conditions in producing the glass-ceramic, or otherfactors.

When a powder or a controlled particle size powder of the glass-ceramicaccording to the invention is used as a porcelain, any known additivessuch as opacifiers, coloring pigments, fluorescent materials or the likemay be added as required, as long as they do not inhibit the effects ofthe invention.

Useful opacifiers include, for example, rutile or anatase TiO₂, SnO₂,ZrSiO₄, CeO₂, and stabilized ZrO₂ (stabilizer=Y₂O₃, CaO, MgO or thelike).

Examples of coloring pigments include Fe₂O₃ pigments, Fe₂O₃—Cr₂O₃pigments, Fe₂O₃—CoO—Cr₂O₃ pigments, PrO₂ pigments, V₂O₅ pigments, CeOpigments, MnO₂ pigments and SnO₂—Cr₂O₃ pigments.

Examples of fluorescent materials include Ce-doped Y₂O₃.

The porcelain mixture containing specific additives is kneaded in aroutine manner using water, modeling liquid (e.g., aqueous PVAsolution), and then applied to or built up on a metal frame. To improvethe workability, a known paste-form kneading agent (such as polyethyleneglycol dimethyl ether, polyethylene glycol with a specific degree ofpolymerization, or the like) may be added to make the porcelain mixtureinto a paste.

When the glass-ceramic powder according to the invention is used as aporcelain for dental prosthesis fabrication, the powder can be used bythe same method as for known porcelains. For example, a coating processconsisting of building up, fusion-bonding and cooling the porcelain onthe surface of a metal frame is repeated several times to form theexternal shape of dental prosthesis. The glass-ceramic according to theinvention undergoes no substantial change in leucite crystal contentwhen fired and thus has a substantially constant coefficient of thermalexpansion, even after the glass-ceramic is subjected to 10 cycles, eachconsisting of (i) vacuum firing defined in JIS T 6515 (manufacturer'sspecific process) and (ii) placing the glass-ceramic in a furnace at apredetermined temperature (for example 600° C.), vacuumizing thefurnace, then raising the temperature in the furnace to 900° C. at arate of 60° C./min, and taking out the glass-ceramic and allowing it tocool in the atmosphere (total firing time=about 1 hour). Further, thefired body obtained in the above manner is highly transparent sinceprecipitation of different types of crystals such as Na—K feldspathiccrystals is inhibited. Therefore, the glass-ceramic obtained accordingto the invention is extremely useful as a porcelain for use on a dentalmetal frame.

Preferably, the metal frame is made of a known precious metal alloy.Examples of precious metal alloys include high karat golds, medium karatgolds, gold-silver-palladium alloys, gold-palladium alloys andsilver-palladium alloys.

In building up and fusion-bonding a powder of the glass-ceramic of theinvention onto the metal frame, an undercoat porcelain is used toconceal the metal color. As the undercoat porcelain, a glass-ceramicpowder containing an opacifier (alias “opaque porcelain”) or a kneadedpaste thereof (alias “paste opaque”) is preferably used. As a topcoatporcelain formed to imitate natural teeth (alias “dentine” or “enamel”),a glass-ceramic powder containing a coloring pigment (which mayoptionally contain a small amount of opacifiers) is preferably used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between the heat treatmenttime for crystallization and the coefficient of thermal expansion in thefired bodies of the glass-ceramic powders obtained in Example 1 andComparative Example 1.

FIG. 2 is a graph showing the relationship between the heat treatmenttime for crystallization and the integrated intensity of X-raydiffraction line in the fired bodies of the above two types ofglass-ceramic powders.

BEST MODE FOR CARRYING OUT THE INVENTION

The following Examples are provided to illustrate the features of thepresent invention in further detail.

Table 1 shows the compositions (wt. %) of the glassy material (1) and ofthe synthetic leucite seed crystals (2) with high purity used in thefollowing Examples and Comparative Examples.

TABLE 1 Leucite seed Glassy material (1) crystals (2) Component G1 G2 G3G4 G5 G6 L1 L2 SiO₂ 64.1 63.9 65.3 63.6 63.8 61.7 55.1 55.1 Al₂O₃ 15.015.1 15.0 15.0 14.7 15.0 23.3 23.3 K₂O 10.3 10.5 11.0 10.8 10.9 12.221.6 21.6 Na₂O 8.5 8.4 6.6 8.5 8.5 9.0 — — MgO 0.3 0.3 0.3 0.3 0.3 0.3 —— CaO 1.2 1.2 1.2 1.2 1.2 1.2 — — B₂O₃ 0.6 0.6 0.6 0.6 0.6 0.6 — —Particle <75 μm <75 μm <75 μm <75 μm <75 μm <75 μm <75 μm 3.5 μm size(average)

EXAMPLE 1

Glassy material sample G1 (99 parts by weight) and leucite seed crystalsample L1 (1 part by weight) shown in Table 1 were uniformly mixedtogether, heat-treated at 850° C. for a given period of time rangingfrom 1 hour to 24 hours for crystallization, cooled and pulverized toobtain glass-ceramic powders according to the present invention.

The obtained glass-ceramic powders had a leucite crystal content rangingfrom 0 to 20.5 wt. %, and the leucite crystals had mean particle sizesof 5 μm or smaller.

Comparative Example 1

Glass-ceramic powders were prepared by following the procedure ofExample 1 except that the leucite seed crystals were not used.Specifically, 100 parts by weight of glassy material sample G1 washeat-treated at 850° C. for a given period of time ranging from 1 to 24hours, cooled, pulverized to obtain comparative glass-ceramic powders.

The obtained glass-ceramic powders had a leucite crystal content rangingfrom 0 to 18 wt. %, and the leucite crystals had mean particle sizes of5 μm or smaller.

Test Example 1

The glass-ceramic powders obtained in Example 1 and Comparative Example1 were molded in a biaxial press. The molded bodies (6 mm×6 mm×25 mm)were placed in a vacuum furnace at 600° C., and then the pressure in thefurnace was reduced to about 6.7 kPa. Subsequently, the molded bodieswere heated to 900° C. at a rate of 60° C./min and maintained at thesame temperature for 1 minute for firing. The pressure in the furnacewas raised to atmospheric pressure, and the fired bodies were cooled toabout 600° C. in the furnace and then allowed to cool outside thefurnace. In the following Table 2, “after 4 times of refiring” means“after subjecting a vacuum-fired body obtained by following the abovefiring schedule to a similar firing schedule (without the reduction ofpressure) four times”. As used hereinafter, the term “4-time refiredbody” means a body obtained by firing a vacuum-fired body four times.

The coefficients of thermal expansion (the average of the valuesobtained at temperatures from 50 to 500° C.) of the obtainedvacuum-fired bodies and 4-time refired bodies were measured using athermal dilatometer.

FIG. 1 is a graph representing the relationship between thecrystallization heat treatment time (h) of each glass-ceramic and thecoefficient of thermal expansion (×10⁻⁶/° C.) of each fired body, invacuum-fired bodies prepared using the glass-ceramic powders obtained inExample 1 and Comparative Example 1 as starting materials and 4-timerefired bodies thereof. As mentioned above, the temperature forcrystallization heat treatment was 850° C.

FIG. 1 reveals that, in the vacuum-fired bodies of the glass-ceramicpowders obtained in Example 1 (Curve 1), the coefficient of thermalexpansion was nearly reached the maximum (13.3×10⁻⁶/° C.) in 2 hours ofthe heat treatment at 850° C., and maintained the same value to thepoint where the heat treatment time was 8 hours. As the heat treatmentwas increased to 12 hours, the coefficient of thermal expansion wassharply decreased to a value less than the coefficient of thermalexpansion of the glassy body before the crystallization heat treatment(9.3×10⁻⁶/° C.).

Also in the 4-time refired bodies of the glass-ceramic powders obtainedin Example 1 (Curve 2), the coefficient of thermal expansion changedslightly as the heat treatment time increased from 2 to 8 hours.

From the results obtained in the vacuum-fired bodies of glass-ceramicpowders prepared by crystallization heat treatment at 850° C. and 4-timerefired bodies thereof, it is apparent that the leucite crystal content(the corresponding coefficient of thermal expansion) in the porcelainaccording to the invention does not substantially change in the abovefiring schedule (which is the same as the actual firing schedule forporcelains).

In contrast, in the vacuum-fired bodies of the glass-ceramic powdersobtained in Comparative Example 1 (Curve 3), the increase in thecoefficient of thermal expansion owing to the heat treatment was small,and the coefficient of thermal expansion increased only to 12.3×10⁻⁶/°C. in 8 hours of the heat treatment. Moreover, the vacuum-fired bodieshad slight turbidity at that point, and were translucent when thecoefficient of thermal expansion reached 12.7×10⁻⁶/° C.

Further, in the 4-time refired bodies obtained using the glass-ceramicpowders of Comparative Example 1 (Curve 4), the coefficient of thermalexpansion greatly changed.

It is clear from the above results that in a glass-ceramic obtained byadding a leucite crystal powder as seed crystals to a glassy materialand heat-treating the mixture for crystallization under specificconditions, the coefficient of thermal expansion increases to a desiredvalue in a short period of time and is maintained at the desired highvalue for a long period of time. As a result, there is a long period oftime before turbidity or opacification occurs.

FIG. 2 is a graph showing the relationship between the heat treatmenttime (h) and the integrated intensity (a.u.) of X-ray diffraction lineof each glass-ceramic fired body, in the case of vacuum-firing theglass-ceramic powders obtained in Example 1 and Comparative Example 1 byfollowing the same firing schedule as described above.

In FIG. 2, Curve 5 represents the intensity of X-ray diffraction line(d₂₁₁) from leucite in the fired bodies of the glass-ceramic powdersprepared in Comparative Example 1; Curve 6 represents the intensity ofX-ray diffraction line (d₂₁₁) from leucite in the fired bodies ofglass-ceramic powders prepared in Example 1; Curve 7 represents theintensity of X-ray diffraction line (d₁₃₀) from high temperature-typeNa—K sanidine crystals in the fired bodies of the glass-ceramic powdersprepared in Comparative Example 1; and Curve 8 represents the intensityof X-ray diffraction line (d₁₃₀) from high temperature-type Na—Ksanidine crystals in the fired bodies of glass-ceramic powders preparedin Example 1.

FIGS. 1 and 2 reveal the following.

The increase in the coefficient of thermal expansion shown in FIG. 1corresponds to the increase in leucite crystal content in the glass.Specifically stated, when no seed crystals are present (FIG. 2; Curve5), leucite crystals begin to precipitate after an induction period ofabout 1 hour. On the other hand, when the seed crystals are added (FIG.2; Curve 6), leucite crystals immediately begin to precipitate withoutan induction period, and the precipitation of the crystals reaches thesaturation point in a short period (about 2 hours) of the heat treatment(in the corresponding Curve 1 in FIG. 1, the coefficient of thermalexpansion nearly reaches the maximum).

Further, as seen from the graph in FIG. 2 (see Curve 7 relating toComparative Example 1 and Curve 8 relating to Example 1), hightemperature-type Na—K sanidine crystals begin to precipitate after aninduction period of about 8 hours, and accordingly notable turbidity andopacification occur in the crystallized glass-ceramic (see Curve 5relating to Comparative Example 1 and Curve 6 relating to Example 1).Similarly, it is understood that, in Curve 1 in FIG. 1, theglass-ceramics obtained by long-time crystallization heat treatment havea coefficient of thermal expansion lower than that of the glass withoutheat treatment (9.3×10⁻⁶/° C.) because of the low thermal expansionproperties of the precipitated high temperature-type Na—K sanidinecrystals.

The above matters reveal the following:

(a) High temperature-type Na—K sanidine crystals, which usually causeopacification in a glass-ceramic of the above type, behave as anapparently stable crystal phase under the heat treatment conditionsemployed in Test Example 1.

(b) In contrast, leucite crystals, which are effective in maintainingthe transparency of the glass-ceramics, precipitate first, but are ametastable crystal phase.

(c) Accordingly, the addition of leucite seed crystals to the glassymaterial is essential to promote the precipitation of leucite crystals(metastable crystal phase), thereby maintaining the transparency of theglass-ceramic which is to be subjected to heated conditions for a longperiod of time, and stabilizing the coefficient of thermal expansion.

(d) It is clear that, when the leucite crystal-containing glass-ceramicobtained by the method of the invention is used as a porcelain, theceramic coating layer formed in a series of fabrication steps for adental prosthesis comprising application/buildup, fusion and cooling ona metal frame is substantially free of opacification and decrease intransparency.

EXAMPLE 2

Glassy material sample G2 (99 parts by weight) and leucite crystalsample L1 (1 part by weight) shown in Table 1 were uniformly mixedtogether, heat-treated for crystallization at 850° C. for 4 hours,cooled and pulverized to obtain a glass-ceramic powder according to theinvention.

The obtained glass-ceramic powder had a leucite crystal content of 18wt. %, and the leucite crystals had a mean particle size of 5 μm orsmaller.

EXAMPLE 3

A glass-ceramic powder according to the invention was prepared byfollowing the procedure of Example 2 except that the crystallizationheat treatment at 850° C. was carried out for 3 hours.

The glass-ceramic powder had a leucite crystal content of 17.5 wt. %,and the leucite crystals had a mean particle size of 5 μm or smaller.

EXAMPLE 4

A glass-ceramic powder according to the invention was prepared byfollowing the procedure of Example 2 except that 98 parts by weight ofglassy material sample G2 and 2 parts by weight of leucite crystalsample L1 were used.

The obtained glass-ceramic powder had a leucite crystal content of 18.5wt. %, and the leucite crystals had a mean particle size of 5 μm orsmaller.

Comparative Example 2

A glass-ceramic powder was prepared by following the procedure ofExample 2 except that only glassy material sample G2 (100 parts byweight) was used.

The obtained glass-ceramic powder had a leucite crystal content of 1.5wt. %, and the leucite crystals had a mean particle size of 5 μm orsmaller.

Comparative Example 3

A glass-ceramic powder was prepared by following the procedure ofComparative Example 2 except that the crystallization heat treatment at850° C. was carried out for 8 hours.

The obtained glass-ceramic powder had a leucite crystal content of 10.5wt. %, and the leucite crystals had a mean particle size of 5 μm orsmaller.

Comparative Example 4

A glass-ceramic powder was prepared by following the procedure ofComparative Example 2 except that the crystallization heat treatment at850° C. was carried out for 12 hours.

The obtained glass-ceramic powder had a leucite crystal content of 17.5wt. %, and the leucite crystals had a mean particle size of 5 μm orsmaller.

EXAMPLE 5

Glassy material sample G6 (99 parts by weight) and leucite crystalsample L1 (1 part by weight) shown in Table 1 were uniformly mixedtogether, heat-treated at 850° C. for 4 hours for crystallization,cooled and pulverized to obtain a glass-ceramic powder according to theinvention.

The obtained glass-ceramic powder had a leucite crystal content of 35wt. %, and the leucite crystals had a mean particle size of 5 μm orsmaller.

Comparative Example 5

A glass-ceramic powder was prepared by following the procedure ofExample 5 except that only glassy material sample G6 (100 parts byweight) was used.

The obtained glass-ceramic powder had a leucite crystal content of 4 wt.%, and the leucite crystals had a mean particle size of 5 μm or smaller.

EXAMPLE 6

Glassy material sample G3 (99 parts by weight) and leucite crystalsample L1 (1 part by weight) shown in Table 1 were uniformly mixedtogether, heat-treated at 850° C. for 2 hours for crystallization,cooled and pulverized to obtain a glass-ceramic powder according to theinvention.

The obtained glass-ceramic powder had a leucite crystal content of 13wt. %, and the leucite crystals had a mean particle size of 5 μm orsmaller.

EXAMPLE 7

Glassy material sample G4 (99 parts by weight) and leucite crystalsample L1 (1 part by weight) shown in Table 1 were uniformly mixedtogether, heat-treated at 850° C. for 2 hours for crystallization,cooled and pulverized to obtain a glass-ceramic powder according to theinvention.

The obtained glass-ceramic powder had a leucite crystal content of 22.5wt. %, and the leucite crystals had a mean particle size of 5 μm orsmaller.

EXAMPLE 8

A glass-ceramic powder according to the invention was prepared byfollowing the procedure of Example 7 except that leucite crystal sampleL2 shown in Table 1 was used in place of leucite crystal sample L1.

The obtained glass-ceramic powder had a leucite crystal content of 23.5wt. %, and the leucite crystals had a mean particle size of 5 μm orsmaller.

EXAMPLE 9

Glassy material sample G5 (99 parts by weight) and leucite crystalsample L1 (1 part by weight) shown in Table 1 were uniformly mixedtogether, heat-treated at 850° C. for 4 hours for crystallization,cooled and pulverized to obtain a glass-ceramic powder according to theinvention.

The obtained glass-ceramic powder had a leucite crystal content of 21.5wt. %, and the leucite crystals had a mean particle size of 5 μm orsmaller.

Test Example 2

Each of the glass-ceramic powders (passed through 200 mesh) obtained inExamples 2 to 9 and Comparative Examples 2 to 5 was molded in the samemanner as in Test Example 1, and made into a vacuum-fired body and4-time refired body by following the firing schedules similar to thoseemployed in Test Example 1. Then, the coefficients of thermal expansionof these fired and refired bodies were measured, and the appearances ofthe 4-time refired bodies were observed by the naked eye.

Table 2 presents these results.

TABLE 2 Coefficient of thermal expansion (10⁻⁶/° C.) Vacuum-fired body4-time refired body Appearance Example 2 13.1 13.0 Transparent Example 313.0 13.1 Transparent Example 4 13.2 13.0 Transparent Example 5 16.516.5 Transparent Example 6 12.1 12.2 Transparent Example 7 14.0 14.1Transparent Example 8 14.2 14.1 Transparent Example 9 13.8 13.8Transparent Comp. Ex. 2 9.8 11.5 Transparent Comp. Ex. 3 11.6 12.8Opaque Comp. Ex. 4 13.0 12.6 Non-transparent Comp. Ex. 5 10.3 13.5Transparent

Table 2 reveals that the vacuum-fired bodies of the glass-ceramicpowders obtained in Examples 2 to 9 had the desired coefficient ofthermal expansion, and that the 4-time refired bodies of theseglass-ceramic powders had a transparent appearance and substantially thesame coefficients of thermal expansion as the corresponding vacuum-firedbodies.

In contrast, the vacuum-fired body and 4-time refired body of theglass-ceramic powder obtained in Comparative Example 2 did not have adesired coefficient of thermal expansion.

As to the glass-ceramic powder obtained in Comparative Example 3, its4-time refired body had a desired high coefficient of thermal expansion,but was unsuitable for practical use owing to its opaque appearance.

The vacuum-fired body of the glass-ceramic powder obtained inComparative Example 4 had a desire coefficient of thermal expansion, butthe 4-time refired body of the glass-ceramic powder had anon-transparent appearance and thus was unsuitable for practical use.

The vacuum-fired body of the glass-ceramic powder obtained inComparative Example 5 did not have a desired coefficient of thermalexpansion.

Test Example 3

Tested were the mechanical and chemical characteristics (describedhereinafter) of a translucent porcelain (Sample 1, α=13.8×10⁻⁶/° C.)prepared by adding coloring agent (=opacifiers, pigments and fluorescentmaterials in the total amount of about 0.3 wt. % of the glass-ceramicpowder weight) to the glass-ceramic powder obtained in Example 9, andfour kinds of commercial translucent porcelains A to D as comparativesamples.

The translucent porcelains used as samples have the followingcrystallographic characteristics.

Sample 1: A porcelain prepared by adding a small amount (about 0.3 wt.%) of coloring agent to the glass-ceramic powder obtained in Example 9(leucite crystal content: 21.5 wt. %)

Sample 2 (commercial porcelain A): A porcelain with a leucite crystalcontent of 20.2 wt. %, which contains a small amount of coloring agentand a small amount of cristobalite as a different type of crystal

Sample 3 (commercial porcelain B): A porcelain with a leucite crystalcontent of 18.3 wt. %, which contains a small amount of coloring agentand a small amount of high temperature-type Na—K sanidine as a differenttype of crystal

Sample 4 (commercial porcelain C): A porcelain with a leucite crystalcontent of 18.9 wt. %, which contains a small amount of coloring agentand a small amount of high temperate-type Na—K sanidine as a differenttype of crystal

Sample 5 (commercial porcelain D): A porcelain with a leucite crystalcontent of 7.4 wt. %, which contains a small amount of coloring agent

Vickers Hardness

Test pieces were prepared from Samples 1 to 5 in the same manner as inthe thermal expansion test in Test Example 2. The test pieces weremirror finished, and the Vickers hardnesses Hv thereof were measured(load=2.9 N, loading time=15 seconds). The results are shown in Table 3given hereinafter.

The test piece prepared from Sample 1 according to the invention had asintered layer with a Vickers hardness Hv of 485, which is close to thatof natural tooth enamel (about Hv 400). Thus, it is presumed that when adental prosthesis fabricated using the porcelain according to theinvention is used in the oral cavity, it causes less abrasion to naturalteeth.

Also, the sintered layer of the test piece prepared from Sample 4 had aVickers hardness relatively close to that of natural teeth.

On the other hand, the sintered layers of the test pieces prepared fromcommercially available Samples 2, 3 and 5 had a Vickers hardness Hv of500 or more. Therefore, dental prostheses fabricated using theseporcelains, when used in the oral cavity, are likely to abrade naturalteeth.

Bending Strength

Using Sample 1 according to the invention and commercially availableSamples 2 to 5, the three-point bending tests defined in JIS T 6516 werecarried out. Table 3 shows the results.

Sample 1 according to the invention had a bending strength of 125 MPa,which is the highest in Table 3. Thus, a dental prosthesis fabricatedusing the porcelain according to the invention, when used in the oralcavity, is not liable to be damaged by occlusion pressure.

Contrastingly, commercially available Samples 2, 3 and 5 had low bendingstrengths. Thus, when dental prostheses fabricated using theseporcelains are used in the oral cavity, they are likely to be broken.

In the porcelain according to the invention, the leucite crystalparticles contained have an average particle size of 5 μm or smaller,and are uniformly dispersed through a porcelain. As a result, theporcelain according to the invention is free of concentration of bendingstress, and has a high bending strength.

TABLE 3 Bending strength Sample Hardness (Hv) (MPa) 1 485 125 2 522 1033 497 96 4 485 74 5 545 110

Solubility Test

Samples 1 to 5 were subjected to solubility tests in a 4% aqueous aceticacid solution, according to JIS T 6516.

All of the test pieces prepared from Samples 1 to 5 had 50% or less ofthe acceptable solubility defined in JIS T 6516 (0.05 wt. %).

INDUSTRIAL APPLICABILITY

In the present invention, leucite seed crystals synthesized in advanceare used as a starting material, so that a leucite crystal-containingglass-ceramic with excellent high temperature stability can be easilyprepared.

In the glass-ceramic obtained by the invention, different types ofcrystals such as Na—K feldspathic crystals start to precipitate longafter the precipitation of leucite crystals reaches the saturationpoint. Therefore, during fusion-bonding to a metal frame, theglass-ceramic of the invention is substantially free of change inleucite crystal content and precipitation of other types of crystalssuch as Na—K feldspathic crystals, and thus has no substantialopacification.

Further, the coefficient of thermal expansion of the glass-ceramicobtained by the invention can be matched with that of a material ofmetal frame to be employed.

Therefore, when the glass-ceramic obtained by the invention is used as aporcelain for metal-ceramic dental restoration, the glass-ceramic doesnot substantially decrease in the coefficient of thermal expansion andis not substantially opacified, during fusion-bonding to a metal framematerial.

The glass-ceramic obtained by the invention is highly transparent andhas the excellent property of being freely colored, which is necessaryto reproduce the color of natural teeth.

Since the glass-ceramic obtained by the invention contains at least 50wt. % of glass phase, the glass-ceramic has excellent sinteringproperties and can be easily degassed during vacuum firing. Further, theglass-ceramic has excellent wettability on metal frames, and a highadhesion strength to frame materials.

The glass-ceramic obtained by the invention is also excellent in variousmechanical characteristics (bending strength, Vickers hardness, etc.)and (chemical stability, etc.).

Moreover, even when a large proportion of opacifiers, inorganic coloringpigments, fluorescent materials or the like is added to a powder orcontrolled particle size powder of the glass-ceramic obtained by theinvention, the resulting mixture is highly capable of being softened andfluidized by heating. This is because the glass-ceramic contains a largeproportion of glass phase. Owing to this excellent property of beingsoftened and fluidized, the amount of color pigment powders to be addedcan be adjusted in a wide range, so as to provide a porcelain foreffectively concealing the color of a metal frame (opaque porcelain), orfor effectively reproducing the color of natural teeth (dentine—dentineporcelain, enamel—enamel porcelain, margin formation—margin porcelain,and restoration and glazing—glaze porcelain).

What is claimed is:
 1. A method for preparing a glass-ceramic containingleucite crystals, comprising the steps of: mixing (1) a glassy materialcomprising 53 to 65 wt. % of SiO₂, 13 to 23 wt. % of Al₂O₃, 9 to 20 wt.% of K₂O and 6 to 12 wt. % of Na₂O, and (2) synthetic leucite seedcrystals comprising 53 to 64 wt. % of SiO₂, 19 to 27 wt. % of Al₂O₃ and17 to 25 wt. % of K₂O; and heat-treating the mixture at 750 to 950° C.for 1 to 5 hours.
 2. A method according to claim 1, wherein thesynthetic leucite seed crystals (2) are at least one member selectedfrom the group consisting of leucite crystals having a theoreticalcomposition and leucite solid solutions containing a SiO₂ componentdissolved therein.
 3. A method according to claim 1, wherein theproportion of the synthetic leucite seed crystals (2) is 0.5 to 3 partsby weight per 100 parts by weight of the glassy material (1).
 4. Amethod according to claim 2, wherein the proportion of the syntheticleucite seed crystals (2) is 1 to 2 parts by weight per 100 parts byweight of the glassy material (1).
 5. A method according to claim 1,wherein the glassy material (1) contains at least one member selectedfrom the group consisting of F and oxides of Li, Mg, Ca, Sr, B, P, Tiand Zr in a total proportion not exceeding 6 wt. %, and wherein thesynthetic leucite seed crystals (2) contain at least one member selectedfrom the group consisting of F and oxides of Li, Na, Rb, Mg, Ca, Sr, B,P, Ti and Zr in a total proportion not exceeding 3 wt. %.
 6. A methodaccording to claim 5, wherein the glassy material (1) contains at leastone member selected from the group consisting of 0 to 2 wt. % of Li₂O, 0to 3 wt. % of MgO, 0 to 3 wt. % of CaO, 0 to 2 wt. % of SrO, 0 to 3 wt.% of B₂O₃, 0 to 2 wt. % of P₂O₅ and 0 to 2 wt. % of F, and wherein thesynthetic leucite seed crystals (2) comprise at least one memberselected from the group consisting of 0 to 5 wt. % of Rb₂O, 0 to 2 wt. %of Na₂O, 0 to 2 wt. % of Li₂O, 0 to 3 wt. % of MgO and 0 to 3 wt. % ofCaO.
 7. A method according to claim 1, wherein the glassy material (1)comprises 61 to 65 wt. % of SiO₂, 14 to 17 wt. % of Al₂O₃, 11 to 16 wt.% of K₂O, 7 to 10 wt. % of Na₂O, 0.3 wt. % or less of Li₂O, 0.1 to 0.6wt. % of MgO, 0.5 to 2 wt. % of CaO, 1 wt. % or less of SrO, 0.3 to 1.2wt. % of B₂O₃, 2 wt. % or less of TiO₂, 0.5 wt. % or less of ZrO₂, 0.5wt. % or less of P₂O₅ and 0.5 wt. % or less of F, in a total proportionnot exceeding 6 wt. %.
 8. A method according to claim 1, wherein both ofthe glassy material (1) and the synthetic leucite seed crystals (2) arepowders having a particle size of 200 mesh or smaller.
 9. A methodaccording to claim 1, wherein the heat treatment is carried out at 800to 900° C. for 1 to 5 hours.
 10. A dental porcelain powder comprising,as a main ingredient, a powder of a glass-ceramic prepared by a methodaccording to claim 1; the dental porcelain powder having a coefficientof thermal expansion which matches that of a dental alloy to beemployed, and being colored so as to reproduce the color of naturalteeth.
 11. A metal-ceramic dental restoration fabricated using aporcelain powder according to claim 10.